I am currently in the process of sanding and then repainting all the doors in my house. Before I started sanding (the doors were white) it was quite easy to find tiny dents/holes or other imperfections. Now I've sanded (door is a salmon pink/gray/yellow all the different paint colors that were on there) and I can still find the dents and imperfections but it is really hard to see them.

Using raking light as advised by @jdv is probably the most widely utilised way to find imperfections in a surface. But you don't have to sweat finding them all now, many good painting guides suggest priming and then filling anyway. And after a uniform, matt, primer has dried it's usually very easy to find imperfections that were unnoticeable previously.
– GraphusAug 1 at 16:24

1 Answer
1

There are two methods I use for finding imperfections like this I want to feather out:

Shine a bright light at a low angle to the material. Imperfections will show up as high contrast reflections or dark shadows. Some places sell hand-held fluorescent tube lanterns specifically for this sort of thing.

Move your hand across the surface. Your fingers will find all sorts of imperfections nearly invisible to the eye.

(1) is used a lot in the high-end car-body finishing industry, and generally in many industries where smoothness needs to be assessed by eye.

Using low-angle light is also very common in surfboard shaping. Often shaping rooms will have lights mounted under cabinet-height shelves so they don't shine into your eyes when you're standing but provide low-angle light to the whole board from anywhere around it.
– SaSSafraS1232Aug 1 at 15:56

I bought a small LED task light that I use for your 1 with a torch (flashlight) at one end and a light bank on one side. These seem very common now, I've seen them for sale in lots of different places. My one is battery powered so no cord to get tangled on anything. It just uses AAs, but because the bulbs are LEDs the batteries last for ages.
– GraphusAug 1 at 16:28